Commercial processes for manufacture of alkylene oxides, e.g., ethylene and propylene oxide, are well known. In a typical process, an alkylene is oxidized over a silver-containing catalyst to the corresponding oxide, which may be recovered or may be further reacted, e.g., with water to form the corresponding glycol. In such oxide manufacturing processes, carbon dioxide and various organic materials are often formed as unwanted by-products.
In a known process for removing the carbon dioxide, the carbon dioxide-bearing stream is scrubbed with an aqueous solution of alkali metal carbonates and/or bicarbonates. Such a process is described in, e.g., U.S. Pat. No. 3,907,969 and Great Britain Pat. No. 1,415,036, the disclosures of which are incorporated herein by reference. The process of that patent involves the use of vanadium and other catalytic materials which would be expensive to continually replenish, as well being undesirable to release to the environment. Accordingly, it is advantageous to provide an improvement to that process whereby the depleted scrubbing stream is treated to remove various contaminants, and is returned to the process.
A method for significantly improving the catalytic oxidation of the alkylene to the corresponding oxide has recently been disclosed in U.S. Pat. Appln. Ser. No. 091352, filed Jul. 14, 1993, the disclosure of which is incorporated herein by reference. This process utilizes silver catalysts of the type comprising at least one efficiency-enhancing salt of a member of a redox-half-reaction pair which are employed in processes in which at least one efficiency-enhancing gaseous member of a redox-half-reaction pair is present (described hereinbelow). When the process of this patent application is combined with the carbon dioxide removal process described above, undesirable nitrate and/or nitrite, as well as various organic, contaminants may be formed.
It has now been found that organic contaminants, as well as inorganic contaminants such as those resulting from processes such as those of the above-cited patent application, can be successfully removed, or optionally controlled in concentration, without significant destruction or depletion of the expensive chemicals required for the carbon dioxide scrubbing procedure, by the method reported below.